A mobile communication system allows each base station or Node-B located in a single cell or sector to communicate with a plurality of user terminals (e.g., user equipments) over a wireless channel environment.
In the case of a multi-carrier system or other systems similar to the multi-carrier system, the base station receives packet traffic from a wired Internet network in the multi-carrier system or other similar systems, and transmits the received packet traffic to each terminal using a predetermined communication scheme.
In this case, the base station determines a downlink scheduling, so that it determines a variety of information according to the downlink scheduling, for example, a user terminal which will receive data from the base station, a frequency area to be used for data transmission to the terminal, and timing information indicating a transmission time of the data to be transmitted to the terminal.
The base station receives packet traffic from the user terminal according to a predetermined communication scheme, and demodulates the received packet traffic, so that it transmits the received packet traffic to the wired Internet network.
The base station determines an uplink scheduling, so that it determines a variety of information according to the uplink scheduling, for example, a user terminal which will transmit uplink data, a frequency band to be used for the uplink data transmission, and timing information indicating a transmission time of the uplink data. Generally, a user terminal having a superior or good channel status is scheduled to transmit/receive data using more frequency resources during a longer time.
FIG. 1 is a conceptual diagram illustrating a time-frequency resource block for use in a multi-carrier system.
Communication resources for use in a multi-carrier system or other similar systems can be largely divided into a time area and a frequency area.
The communication resources can be defined by resource blocks. Each resource block includes N sub-carriers and/or M sub-frames, and is configured in units of a predetermined time. In this case, N may be set to “1”, and M may also be set to “1”.
A single square of FIG. 1 indicates a single resource block. A single resource block uses several sub-carriers as a single axis, and uses a unit of a predetermined time as another axis.
A base station in a downlink selects a user terminal according to a predetermined scheduling rule, allocates one or more resource blocks to the selected user terminal. The base station transmits data to the selected user terminal using the allocated resource blocks.
According to uplink transmission, the base station selects the user terminal, and allocates one or more resource blocks to the selected user terminal according to a predetermined scheduling rule. The user terminal receives scheduling information, indicating that a predetermined resource block has been allocated to the user terminal itself, from the base station, and transmits uplink data using the allocated resource.
Although data has been transmitted according to the scheduling rule, the data may be unexpectedly damaged or lost during the transmission process. In this case, there are proposed a variety method for controlling the faulty or erroneous operation, for example, an automatic repeat request (ARQ) scheme and a hybrid ARQ (HARQ) scheme, etc. The confirmation of the faulty or erroneous operation according to the above-mentioned two schemes is operated in frame units. Data transmitted during the frame unit is hereinafter referred to as a frame.
The ARQ scheme waits for transmission of the ACK signal after transmitting a single frame. If a reception end correctly receives data of the frame, it transmits the ACK signal. However, if an unexpected error occurs in the frame, the reception end transmits a negative-ACK (NACK) signal, and deletes the received erroneous frame from its own buffer.
If the transmission end receives the ACK signal, it transmits the next frame. Otherwise, if the transmission end receives the NACK signal, it retransmits the frame.
The HARQ scheme allows the reception end to transmit the NACK signal to the transmission end on the condition that the received frame cannot be demodulated. However, differently from the ARQ scheme, the HARQ scheme does not delete the pre-received frame from the buffer, and stores the pre-received frame in the buffer for a predetermined period of time. Therefore, if the above-mentioned frame is re-transmitted, in the HARQ scheme the reception end combines the pre-received frame with a re-transmitted frame, thereby it could increase the success rate of data reception.
In recent time, many users prefer to the HARQ scheme to the basic ARQ scheme.
There are a variety of types in the HARQ scheme. For example, the HARQ scheme can be classified into a synchronous HARQ scheme and an asynchronous HARQ scheme.
If initial transmission of data fails, the synchronous HARQ scheme is designed to perform the next retransmission of data at a timing point determined by a system. For example, if it is assumed that the retransmission timing point is set to a fourth time unit after the initial transmission failure occurs, there is no need to additionally indicate the fourth time unit because the retransmission timing between the base station and the user terminal is pre-engaged.
In other words, if the transmission end of data receives the NACK signal, it re-transmits the frame every fourth time unit until receiving the ACK signal.
In the meantime, the asynchronous HARQ scheme is performed by the newly-scheduled retransmission timing and the additional signal transmission. In other words, a timing point at which the previously-failed frame is re-transmitted is variable with a variety of factors such as a channel status.
The HARQ scheme can be classified into a channel-adaptive HARQ scheme and a channel-non-adaptive scheme according to information indicating whether a channel status is reflected in allocation of resources used for retransmission.
The channel-non-adaptive HARQ scheme (also called a non-adaptive HARQ scheme) enables resource blocks used for retransmission, and a MCS (Modulation and Coding Scheme) level defining frame modulation and coding methods to be operated according to a specific scheme predetermined by initial transmission.
The channel-adaptive scheme (also called an adaptive HARQ scheme) allows the above-mentioned resource blocks and the MCS level to be variable with channels status information.
For example, according to the channel-non-adaptive HARQ scheme, a transmission end transmits data using eight resource blocks during the initial transmission, and then re-transmits the data using the same eight resource blocks irrespective of a channel status acquired by retransmission of the data.
On the other hand, according to the channel-adaptive HARQ scheme, although data is initially transmitted using 8 resource blocks, the data may also be re-transmitted using eight or less resource blocks or eight or more resource blocks according to the next channel status as necessary.
According to the above-mentioned classification, the HARQ scheme may have four combinations of the HARQ schemes. According to unique characteristics of the above-mentioned schemes, the most preferred combinations of the HARQ schemes are an asynchronous channel-adaptive HARQ scheme, and a synchronous channel-non-adaptive scheme.
Generally, the asynchronous channel-adaptive HARQ scheme adaptively changes a retransmission timing point and the amount of used resources to others according to a channel status, so that it can maximize the retransmission efficiency. In the meantime, the synchronous channel-non-adaptive HARQ scheme has an advantage in that there is almost no overhead because the retransmission timing and the resource allocation for retransmission are pre-engaged in a system.